1. Field of the Invention
The present invention relates to a method of producing a nitride semiconductor crystal by a metal organic chemical vapor deposition process, a nitride semiconductor crystal produced by such a method, and a semiconductor device provided with such a nitride semiconductor crystal.
2. Description of Related Art
Nitride semiconductor materials composed of nitrides of group III elements (such as B, Al, Ga, and In) find applications in electronic devices, such as light receiving and emitting devices (LEDs, semiconductor lasers, photodiodes, superluminescent diodes, phototransistors, solar cells, etc.), electronic devices other than light receiving and emitting devices (diodes, transistors, bipolar electronic devices, unipolar electronic devices, integrated devices, etc.), spintronics devices, photocatalyst devices, and electron tubes, and in functional materials, such as semiconductor substrates, laser media, magnetic semiconductors, phosphors (luminescent, i.e. phosphorescent and fluorescent, materials). Producing such devices and functional materials requires growing a nitride semiconductor crystal while controlling its electrical conductivity and chemical composition.
Inconveniently, however, with nitride semiconductor materials, p-type conductivity still cannot be controlled sufficiently, and carrier concentration cannot be increased sufficiently. Ordinarily, the p-type conductivity of a nitride semiconductor is controlled by use of Mg as an acceptor dopant. However, an attempt to raise carrier concentration, for example, to the order of 1018/cm3 (atoms per cubic centimeter) ends in an acceptor dopant activation rate of the order of several percent, and it is difficult to raise carrier concentration further.
One solution to this problem is the simultaneous doping technology disclosed in Japanese Patent Application Publication No. H10-101496 (hereinafter Patent Document 1). Patent Document 1 discloses a technology of growing a GaN crystal by use of atomic N+ gas obtained by irradiating a N2 gas source as a source material of nitrogen with electromagnetic waves, wherein doping with Si or O as a donor dopant and doping with Mg or Be as an acceptor dopant proceed simultaneously.
In the technology disclosed in Patent Document 1, dopant source materials are supplied, in the form of an atomic beam, onto a substrate so as to be eventually introduced into a GaN crystal. Here, doping with an acceptor dopant and doping with an donor dopant proceed simultaneously with the atomic ratio of the former to the latter at 2:1. A p-type acceptor (for example, Mg+Ga) and an n-type donor (for example, Si−Ga) introduced into the GaN substrate are charged positively and negatively respectively, and by making a pair, come to have a stable electrostatic energy. Adding another acceptor atom to the pair enables it to act as an acceptor effectively. This helps achieve high-concentration doping, and helps obtain a high carrier concentration, resulting in efficient activation as an acceptor.
In ordinary p-type doping, where the p-type conductivity of a nitride semiconductor is controlled by use of Mg as an acceptor dopant, as mentioned above, it is difficult to raise carrier concentration. Moreover, the dopant level is typically so deep as to exceed 100 meV, and this inconveniently results in a low activation rate at room temperature and large temperature dependence in resistivity and carrier concentration.
The technology disclosed in Patent Document 1 mentioned above is not for crystal growth by an ordinary MOCVD (metal organic chemical vapor deposition) process using a hydride such as ammonia as a source material of nitrogen. This crystal growth method using a hydride such as ammonia as a group V source material gas is superior to other technique in overall terms from the viewpoints of crystal quality, controllability, reproducibility, and productivity, and is therefore applied to mass production of various devices. However, Patent Document 1 does not disclose a technology for simultaneous doping that is effective in such a growth technique.
Even if the simultaneous doping of which the principle is proposed in Patent Document 1 is applied to an MOCVD crystal growth process, it is still difficult to obtain a high carrier concentration and a high activation rate stably. It is also difficult to produce a p-type semiconductor crystal at a shallow dopant level stably.
In a nitride semiconductor crystal, the atomic density of a group III element or a group V element is around the middle of the order of 1022/cm3, and this number is the number of acceptor sites that an acceptor atom can take. On the other hand, the range within which acceptor concentration needs to be controlled in a p-type layer in a semiconductor device is about 1017/cm3 to 1020/cm3, and thus acceptor atoms are distributed in the crystal in a state sparsely substituting acceptor sites, with one acceptor atom for several hundred acceptor sites at most and, in an ordinarily used range up to the middle of the order of 1022/cm3, with one or less acceptor atom for 10000 acceptor sites.
The crux of the principle disclosed in Patent Document 1 is that two acceptor atoms and one donor atom interact with one another in the crystal to stabilize. Accordingly, they need to be disposed in neighboring sites in the crystal. However, in an ordinary MOCVD crystal growth process using a hydride gas as a source material of nitrogen, even when an acceptor source material (in the case of Mg, i.e. the same dopant species as in Patent Document 1, its source material gas is that of an organic metal compound of Mg) and a donor source material (in the case of Si, i.e. the same dopant species as in Patent Document 1, its source material gas is a hydride of Si) are introduced into a growth machine with such control that they are introduced into the crystal in a predetermined atomic ratio, since acceptor atoms (or donor atoms) are distributed extremely sparsely as mentioned above, they are not conveniently introduced in such a way that two acceptor atoms and one donor atom are disposed at neighboring sites.
Acceptor atoms and donor atoms tend to be introduced in a dispersed fashion in the crystal, and thus the principle of stabilization proposed in Patent Document 1 is very unlikely to take effect. Thus, the technology is far from enabling efficient doping. Incidentally, no particular attention is paid to this problem, either, in the technology disclosed in Patent Document 1 of growing a GaN crystal by use of atomic N+ gas obtained by irradiating a N2 gas source as a source material of nitrogen with electromagnetic waves. Thus, the technology is supposed to suffer from a similar problem.